The present invention is in the field of electrical heating and particularly relates to electrical heating units of the so-called integral element type, comprising a glass or other ceramic heating plate or block, which plate or block is heated by an electrical heating element indirectly bonded thereto. Such heating units are particularly useful for electrical cooking ranges, hot plates, and other electrical heating appliances.
U.S. Pat. No. 3,086,101 discloses an electrical heating unit comprising a glass plate having an electrical heating element in physical contact with the lower surface thereof. This unit may optionally include an alumina coating between the element and the plate to prevent chemical interaction therebetween at elevated temperatures.
U.S. Pat. No. 3,067,315 discloses an electrical heating unit of improved heating characteristics comprising a high silica glass plate having directly bonded to the lower surface thereof a thin noble metal film which acts as the electrical heating element of the unit. However, supporting plates having decreased optical transparency and higher strength, particularly higher impact strength, are desired.
Since the discovery of the so-called glass-ceramic family of ceramic materials, such as described in U.S. Pat. No. 2,920,971, electrical heating units comprising glass-ceramic plates heated by electrical heating elements have been introduced into commerce. The strength, low porosity and excellent thermal properties of certain of these glass-ceramic materials have provided electric ranges and other electrical heating units of excellent appearance and cleanability. Up to the present time, however, electrical heating units comprising glass-ceramics have generally been of the discrete element type, such as described in U.S. Pat. No. 3,889,021 and British Pat. No. 1,391,076, wherein the electrical heating element is not directly bonded to but is simply in close physical contact with or proximity to the glass-ceramic plate to be heated. Integral element heating units offer substantial advantages in heating efficiency, but numerous problems are associated with the development of such units.
One of the most important requirements of a glass-ceramic material to be utilized as a burner plate for an electrical heating unit is high strength. Such plates may be subjected to heavy impacts in use and the cost of replacement of the entire plate upon breakage is prohibitive. Glass-ceramic materials normally exhibit higher modulus of rupture strengths than glasses; hence glass-ceramic electrical heating units of the discrete electrical element type typically exhibit adequate resistance to breakage on impact.
Among the glass-ceramic materials presently employed in the fabrication of electrical heating units such as electric ranges are lithium aluminosilicate glass-ceramics of the beta spodumene type or the beta spodumene-beta eucryptite type. Such glass-ceramics exhibit high strength, low thermal expansion, excellent thermal stability and good appearance and cleanability.
However, the use of lithium aluminosilicate glass-ceramics of these types in high temperature electrical applications where voltages are to be directly applied to elements in contact with the glass-ceramic plate requires that a high resistivity electrical barrier be interposed between the plate and the electrical elements, since the high temperature electrical resistivity of lithium aluminosilicates is rather low. Accordingly, a high-resistivity ceramic coating such as, for example, a cordierite coating, is applied to the lithium aluminosilicate glass-ceramic prior to the attachment of electrical heating elements thereto.
We have discovered that serious strength deterioration is encountered in presently available lithium aluminosilicate glass-ceramic burner plate materials upon the application of ceramic electrical barrier layers thereto for the purpose of providing a base for an integral electrical heating element. This problem is apparently related to physical and/or chemical interactions occuring between glass-ceramic substrates and the coating materials applied thereto. These interactions may occur when the base plate and layer are heated, either during the application of the coating material or during the operation of the unit. Thus glass-ceramic burner plate material exhibiting sufficient modulus of rupture strength for use in conventional thicknesses for discrete element electrical heating units may exhibit insufficient strengths following the application of insulating ceramic coating constituents thereto,
This problem is apparently not limited to units comprising electrically-insulating ceramic coatings, but may also occur when other ceramic, metallic, or cermet compositions are directly bonded to the glass-ceramic surface. On the other hand, strength deterioration is normally not observed when superficially adhering coatings are applied. It therefore appears that the difficulties of directly bonding coating compositions to lithium aluminosilicate glass-ceramic plates stem from physical and/or chemical incompatibilities between the plate materials and the ceramics, metals or cermets to be bonded thereto.